Candidiasis is the most common fungal infection, with an estimated 63,000 episodes of invasive candidiasis per year occurring in the United States, with a cost estimated at $2-4 billion. Although a diverse selection of pathogenic fungi have been isolated from clinical samples, the yeast species of the genus Candida are the predominant cause of opportunistic fungal infections, with Candida albicans the prevalent pathogen, causing an estimated 40% of cases of fungemia. For the last decade, Candida infections have been effectively treated with caspofungin, a major antifungal of the echinocandin class that interferes with cell wall synthesis. As antici- pated based on increased use of this drug, the number of reports of infections with caspofungin-resistant strains has increased, from 0.5% in 2001 to 3.1% in 2009, and so caspofungin resistance is expected to be- come a major concern in the near future. However, in contrast to the numerous and well-understood mecha- nisms of C. albicans resistance to fluconazole, little is known regarding the mechanism(s) of resistance to cas- pofungin. There is one recognized mechanism of caspofungin clinical resistance that involves point mutations in the FKS1 (orf19.2929) gene encoding a subunit of 1,3-?-D-glucan synthase required for normal synthesis of glucan, a major component of the cell wall. Formation of FKS1 mutations is always associated with therapeutic failure. Many other isolates from patients contain remodeled cell wall and show increase of resistance to cas- pofungin in laboratory experiments. This co-called cell wall salvage mechanism is associated with reversible alterations that strengthen the cell wall, particularly increased chitin. Our own data show that laboratory resistance to caspofungin can be attained via multiple molecular mech- anisms. When culturing C. albicans in the presence of caspofungin, we have observed the emergence of re- sistant strains monosomic for Ch5, indicating that negative regulators of resistance are resident on this chro- mosome. Monosomic Ch5 leads to decreased glucan and increased chitin in the cell wall. Cloning and charac- terization of the Ch5-linked genes involved in this regulator mechanism will help elucidate key molecular pathways of caspofungin resistance. Moreover, in preliminary studies, we have obtained evidence that Ch5 monosomy involves two-fold downregulation of the FKS1 (see above) or GSL2 genes, residing on Ch1 and ChR, respectively, and required for normal synthesis of cell wall glucan. Based on our findings, we hypothesize that Ch5 carries multiple genes required for normal synthesis of cell wall components. Loss of one Ch5 leads to increased laboratory resistance to drugs of the echinocandin class. In support of this hypothesis, our initial analyses have revealed two candidate genes on Ch5 that are likely to be involved in the mechanism of caspofungin resistance in monosomic strains. Here, we propose to initiate the study of the genes that encode negative regulators of caspofungin resistance. Our findings will be of high significance to clinicians and researchers investigating the phenomenon of drug resistance in C. albicans.